Methods for producing molded plastic articles, which are useful for example for automotive or furniture components or entry way doors, are well known in the art. Typically, such molded plastic components may be manufactured by processes commonly known as "resin transfer molding" (RTM), or "reinforced reaction injection molding" (RRIM), in which a fiber reinforced mat or preform is placed into a mold cavity into which reactive polymeric precursor materials are subsequently injected. The polymeric precursor materials react in situ to form a molded plastic article having fiber reinforcements throughout. Various polymeric precursor materials may be employed, such as for example those which cure in situ to prepare polyurethanes, polyesters, or crosslinked epoxy resins.
Where an aesthetically pleasing surface finish is important, however, the above mentioned processes pose a problem known generally in the art as fiber readout, which is observed at the surface of the fiber reinforced molded plastic article. The readout problem is caused by the presence of continuous fibers near the surface of the molded plastic article. When a reinforced plastic article is molded, and the mold is subsequently parted, the cured viscoelastic polymeric material located at the surface of the article adjacent to a fiber will shrink, causing the surface of the article to reveal the protruding contours of those fibers immediately below the surface of the molded plastic article.
Another contributing factor producing fiber readout is the differential thermal shrinkage that exists between the fibers and the resin. For example, the coefficient of thermal expansion for polymers is generally at least one order of magnitude greater than the coefficient of thermal expansion for glass fibers. As a result when a molded plastic article, utilizing a glass fiber mat or preform, is removed from a mold and allowed to cool to a lower temperature, the glass-rich regions shrink less than the resin-rich regions, which thereby causes the formation of protuberances over the fibers near the surface. It would be desirable to form a fiber reinforced molded plastic article, by the RTM or RRIM process without this fiber readout defect at its surface.
Reaction injection molded polyurethanes are useful polymeric precursor materials for RTM and RRIM processes. U.S. Pat. No. 3,655,597 discloses reacting polyisocyanate, polyol, blowing agent, diamine and catalyst for the manufacture of molded polyurethane components. A detailed description of the RIM process may be found in Prepelka and Wharton "Reaction Injection Molding in the Automotive Industry," Journal of Cellular Plastics, vol. II, no. 2, 1975. The RIM process also has been advantageously employed in the so-called "encapsulated automotive glazing" manufacturing process, wherein a polyurethane gasket is formed around the perimeter of a glazing unit of transparent material such as glass. See U.S. Pat. No. 4,561,625. Resin transfer molding processes utilizing thermoset polyester resins and epoxy resins, are generally disclosed in U.S. Pat. Nos. 4,405,538, issued Sept. 20, 1983, and 4,581,393, issued Apr. 18, 1986, respectively
The processes described are useful for manufacturing for example entry way doors having aesthetically pleasing surface finishes which may be subsequently painted or stained. The prior art generally discloses two methods for preparing doors with surfaces free of fiber readout defects.
U.S. Pat. No. 4,676,041, issued June 30, 1981, discloses encapsulating a core wrapped in glass fiber with a liquid catalyzed resin material which impregnates the glass fibers and simultaneously forms the outer skin or shell of the door. In order to prevent fiber readout at the surface of the door, however, the walls of the mold are first coated with the same liquid catalyzed resin material. This is commonly referred to as a "gel coat" process and insures that the surface of the molded plastic article is devoid of glass fibers.
U.S. Pat. No. 4,550,540, issued Nov. 5, 1985, discloses the use of sheet molding compound (SMC) for preparing the exterior surfaces of a compression molded door. The outer surface of the SMC prepreg is essentially devoid of glass fibers to a predetermined depth before the material is compression molded over a central core.
Other prior art exists disclosing methods for preparing surface finishes free from fiber readout defects during plastic article molding operations.
U.S. Pat. No. 4,714,575, issued Dec. 22, 1987, discloses a method for injecting a high viscosity polyurethane resin into the region above a glass fiber preform, while simultaneously injecting a low viscosity polyurethane resin into the region below the glass fiber preform. The low viscosity material penetrates the porous mat to reinforce it, and the porous characteristic of the mat prevents the high viscosity material from entering. In this fashion, a finish surface, produced by the high viscosity resin, may be prepared which excludes the glass fibers and therefore prevents fiber readout.
U.S. Pat. No. 4,664,862 issued May 12, 1987, discloses a method for mechanically restricting glass fibers from the surface of a molded plastic article, by forming one of the mold cavity surfaces with equally spaced closely packed valleys having a truncated pyramid shape. A molded plastic article having a textured surface is thereby produced wherein the glass fibers are excluded from the immediate vicinity of the textured surface This process, however, cannot be used to prepare molded plastic articles having a smooth surface free from fiber readout.
U.S. Pat. No. 4,668,460, issued May 26, 1987, discloses a process for partially curing a molded plastic article within a compression mold, and thereafter injecting a separate layer of a polymeric material onto the surface of the partially cured article without reopening the mold. The secondly injected polymeric material flows evenly over the partially cured article, and thereby provides a finish layer of cured resin containing no glass fibers.
U.S. Pat. No. 4,267,142, issued May 12, 1981, discloses a process wherein a pre-formed plastic structure is placed into a female mold member having a mating contoured surface, and thereafter an SMC prepreg is compression molded against the pre formed plastic structure. This results in a molded plastic article presenting the pre-formed plastic structure as the finished surface, adhered to the SMC prepreg which contains the glass fibers. The glass fibers are thereby isolated from the surface of the finished molded plastic article.
Finally, U.S. Pat. No. 4,781 876, issued Nov. 1, 1988, discloses a process wherein polyurethane resin is injected into a mold cavity containing glass fiber or a glass fiber preform and allowed to partially cure. Thereafter, the mold is opened and a quantity of a coating material is injected onto the hot surface of the partially cured substrate to form a top layer. Next, pressure is applied, to cause the coating material to fill the valleys between the protruding fibers and to flow evenly over the entire surface of the premolded article, thereby preparing a finished surface devoid of glass fibers.